1. Field of the Invention
The present invention relates generally to measuring the quality, or wetness, of steam. More particularly, the present invention relates to an optical system and method for determining the quality of a flow of steam by using laser diode emitter and photo diode detector devices to measure radiant energy absorption due to liquid water in the flow of steam.
2. State of the Art
The efficiency and lifetime of components in steam systems are affected by the quality, or wetness, of the steam. Steam quality is defined as the percentage of the total fluid mass of steam that is in the vapor phase. “Dry” steam or steam of 100% quality consists solely of water in the vapor phase. Steam quality of less than 100% indicates the steam contains a portion of water in the liquid phase corresponding to the reduction in quality percentage. Such steam is referred to as “wet” steam. The portion of water in the liquid phase is often due to condensation caused by temperature drops or other energy losses at points throughout a steam system. Excessive steam washing to remove particulates or reduce concentrations of chloride or other impurities to levels that are non-damaging to system components can also introduce excess moisture. The resulting wet steam carries water droplets that can corrode system components such as turbine blades or piping, eventually resulting in failure thereof. In addition, entrained water droplets often contain solids that can deposit on turbine surfaces, adversely affecting the flow stream and turbine efficiency as well as potentially causing imbalance and necessitating cleaning operations. Wet steam also contains less usable energy than dry steam, translating into a loss in mechanical performance. Control of steam quality during operation can be used to anticipate and correct these problems. Accordingly, online or “real time” monitoring and feedback of steam quality is important for diagnostic purposes to increase operating efficiency and to reduce equipment maintenance and replacement costs.
Historically, calorimeters have been used to measure steam quality, but difficulties with sensitivity, accuracy and range limit their suitability for use in many applications. Moreover, calorimeters in most cases require slip stream installations and the insertion of multi-port probes which may disrupt steam flow and result in measurement errors due to nonrepresentative sampling. Calorimeters also exhibit slow response times, making them unsuitable for continuous, real time monitoring. Such continuous monitoring is important if quality measurement of steam is to be used for diagnostic and failure detection purposes. Other attempts to measure steam quality while avoiding the drawbacks of calorimetry have been developed using optical methods. By observing changes in light or other radiant energy passed through a flow of steam, and comparing the changes with known effects of water droplets on the various wavelengths transmitted, a determination of the percentage of liquid water contained in the steam can be made.
In one optical technique, steam quality is measured by comparing the intensity (I) of light passed though a flow of wet steam containing water droplets in suspension to the intensity (Io) of light passed through a flow of dry steam which contains no water droplets. In the wet steam, the light is scattered by the droplets based on the ratio of droplet diameter and the wavelength of the light, thereby reducing its intensity. Under the Mie scattering theory derived from Maxwell's equations, the ratio of intensities (I/Io) can be used to deduce droplet size and distribution, and thereby the amount of liquid water in a given flow of steam. U.S. Pat. No. 6,128,079 to McClosky et al. and U.S. Pat. No. 4,137,462 to Wyler disclose typical examples of this scattering measurement technique, wherein a probe is inserted into a passageway for transmitting a beam of light across a flow of steam and reflecting it back to a photo detector. The invasive nature of this approach raises concerns with respect to the effect of a probe on the flow of steam, which may reduce the accuracy of the scattering measurements. Moreover, scattering of light outside of the measurement area may be reflected back from other surfaces within the passageway, generating noise which must be compensated for by a signal offset.
Another optical technique that has achieved some success is to measure a drop in radiant energy intensity due to absorption by water droplets, rather than scattering. U.S. Pat. No. 4,862,001 to Dowling et al., the disclosure of which is incorporated herein by reference, discloses a steam quality monitor using this technique. Two windows are included on opposite sides of a steam pipe. A source located outside of the steam pipe passes a beam of IR energy through the windows to a detector located on the opposite side of the steam pipe. The loss of IR energy due to absorption by water droplets in the steam is then determined by comparing the energy of the IR beam passed through the steam to one sent from the source to a detector without passing through steam. Using Beer's law, which relates the amount of energy absorbed to the mass of water droplets in a given volume of steam, the steam quality can be calculated. While well known in the prior art, this technique has not been widely used due to the cost and complexity of the instruments required. Large-scale, high-temperature broadband IR sources are required to provide sufficient transmission as the IR energy rarely has the intensity and beam quality necessary for accurate measurements. Broadband IR sources are not collimated and as a result do not present a consistent beam cross section through the measured volume of steam. Since the IR beam may spread quickly, its intensity cross section during transmission across the measured volume changes as a function of the path length. Accordingly, such systems often lack the sensitivity and stability to perform measurements under operating conditions of interest.
What is needed, therefore, is an optical system for measuring steam quality that is sensitive to slight variations in steam quality during real time monitoring while being of a compact and noninvasive nature.